In U.S. Pat. No. 3,485,521 there is illustrated a seal comprising a seal body of plastic material, said body having a pair of cavities, and a U-shaped wire shackle, each leg of which is provided with a reversely bent end for insertion into a body cavity, so that the shackle cannot be disengaged from the body without evidence of such attempt at pilferage being visible, such as rupture of the shackle or protrusion of one of the shackle ends from the surface of the body.
Although such devices are widely used and have been found generally satisfactory in providing a reliable seal, it has been found that in rare instances a leg of the wire shackle can be removed from the seal body without leaving evidence of tampering that is readily noticeable. Also, it is difficult for an inspector to detect, by visual examination, that the seal is properly closed, and the effectiveness of the closure can be assured only by applying a tensile force to the seal, which testing force itself might cause rupture of the seal.